1. Technical Field
This invention relates to a method for production of a stacked battery. More particularly, the present invention relates to a method for production of a stacked battery that is suitable for production of a stacked lithium-ion battery where a plurality of current collection tabs of positive electrodes and negative electrodes are connected in parallel.
2. Related Art
Lithium ion batteries have been used in portable appliances such as portable telephones and digital cameras to meet the demand for downsized high capacity power sources. Additionally, lithium ion batteries are being used as high energy density power sources having no memory effect for electric bicycles and electric tools. Power sources of such portable appliances are required to be downsized and accordingly lithium ion batteries are required to be designed as downsized and flat batteries.
Besides, stacked lithium ion batteries formed by laying one on the other a plurality of plate-shaped positive electrodes and negative electrodes with separators interposed between them, connecting in parallel current collection tabs that are connected respectively to them and being covered by a flexible film casing that provides advantages from the viewpoint of energy density of battery are also being used.
FIGS. 13A and 13B of the accompanying drawings schematically illustrate a known stacked lithium-ion battery before covered by a flexible film casing.
FIG. 13A is a schematic perspective view of the stacked battery and FIG. 13B is a perspective view thereof, illustrating how it appears after welding the current collection tabs and the lead terminals.
Referring to FIGS. 13A and 13B, a plurality of positive electrodes 4 and a plurality of negative electrodes 6 are laid alternately one on the other as so many plates with separators 5 interposed between them to produce a stacked member 2 that operates as a battery element.
As shown in FIG. 13B, the positive electrode current collection tabs 7 and the negative current collection tabs 8 are bonded respectively to a positive electrode lead terminal 9 and a negative electrode lead terminal 10 at respective junctions 11, 12. These components are covered by a flexible film casing (not shown).
Normally, the positive electrode current collection tabs 7 and the negative electrode current collection tabs 8 are bonded respectively to positive lead terminal 9 and negative lead terminal 10 by ultrasonic welding. Each of them is pinched by an anvil and a welding chip and ultrasonically welded. When connecting the current collection tabs and the lead terminals, the welding conditions vary according to the number of current collection tabs that are put together and hence it is conventionally necessary to determine the optimum conditions of the welding apparatus each time the number of current collection tabs is changed. Additionally, in the case of a stacked lithium-ion battery, the thickness of a single current collection tab is as small as several micrometers to tens of several micrometers so that current collection tabs can be cut and broken when large welding energy is input for an increased number of current collection tabs that are to be welded. In other words, there arises a problem that current collection tabs are apt to be cut and broken when a large number of tabs are welded collectively.
A battery where a plurality of lead wires are connected to current collectors is proposed in JP-A-9-213299. The above-cited patent document discloses that at least three connection surfaces are formed to connect lead wires to a current collection terminal and the lead wires of an electrode plate are connected to the respective connection surfaces to prevent the electrode plate from being damaged as a result of forcibly bending lead wires.
Since the required optimum welding conditions vary as a function of the number of current collection tabs to be welded when welding positive and negative electrode current collection tabs to lead terminals in known stacked lithium ion batteries, it is conventionally necessary to determine the optimum conditions of the welding apparatus each time the number of current collection tabs is changed. Additionally, the strength for withstanding welding such as ultrasonic welding of a current collection tab whose thickness is as small as several micrometers to tens of several micrometers is obviously limited and hence there arises a problem that particularly the endmost current collection tab can highly probably be cut and broken in a welding process when the number of current collection tabs becomes large.
Thus, the present invention provides a method for production of a stacked battery with which the current collection tabs can be welded stably. More particularly, it is an object of the present invention to provide a method for production of a stacked battery that does not require any study of the optimum welding conditions each time the number of current collection tabs to be stacked is increased and makes it possible to stably weld current collection tabs without cutting and breaking any of the current collection tabs when the number of current collection tabs is increased.
Another object of the present invention is to provide a method for production of a stacked battery that allows the number of layers of a stacked member to be increased without changing the welding conditions of welding thin belt-shaped positive electrode current collection tabs or negative electrode current collection tabs to a lead terminal for stacked batteries of a number of different types whose number of layers of positive electrodes and negative electrodes for forming the stacked member varies.